Etching solution

ABSTRACT

WHEN CERTAIN MASKED ALLOYS ARE CHEMICALLY ETCHED, AN UNDERSIRABLE EFFECT COMMONLY REFERRED TO AS &#34;CHANNELLING&#34; IS OFTEN ENCOUNTERED. &#34;CHANNELLING&#34; IS CHARACTERISED BY AN AREA OF INCREASED ATTACK IN THE IMMEDIATE VICINITY OF THE BOUNDARY BETWEEN THE MASKED AND EXPOSED AREAS OF THE ALLOY. THE PRESENT INVENTION RELATES TO AN ETCHING SOLUTION COMPRISING A DISPERSION OF FINELY DIVIDED PARTICLES WHICH MINIMISES THIS EFFECT.

States Patent Cl. 1 Claim ABSTRACT OF THE DISCLOSURE When certain masked alloys are chemically etched, an undesirable elrect commonly referred to as channelling is often encountered. Channelling is characterised by an area of increased attack in the immediate vicinity of the boundary between the masked and exposed areas of the alloy. The present invention relates to an etching solution comprising a dispersion of finely divided particles which minimises this effect.

The invention relates. to etching solutions and more particularly to etching solutions for etching surfaces which are partially masked so as to achieve localised etching.

One of the problems encountered in the etching of partially-masked surfaces, is;the formation of a fillet radius at the boundary between the masked and exposed areas 9f .thesurface. The fillet radius extends-under the maskant to a distance approximately equal to the depth of etch. In, order to compensate for this effect it is normal practice to arrange the maskant so that it overlaps the areas to be etched by a distance approximately equal to the intended depth of etch.

. Unfortunately certain alloy surfaces when etched in this fashion exhibn an undesirable effect commonly referred to as channelling. Channelling is characterised by an area of increased attack in the immediate vicinity of the fillet radii, thus resulting in localised areas of stress concentration which in turn may affect the fatigue properties of the etched article.

It has been proposed to reduce channelling by changing the concentration of the etching solution or by careful positioning of the article to be etched in the etching solution. Whilst these measures do reduce channelling to some extent, they do not completely eliminate the effect. It has also been proposed to reduce chann'elling by the addition of Wetting agents to the etching solution, but unfortunately the use of such agents is not possible in strongly acidic solutions.

It is an object of this invention therefore to provide a wide range of etching solutions of differing concentrations which substantially reduce or eliminate channe'lling.

:. According to the present invention, an etching solution comprises a dispersion of finely divided particles, said particles being'inert to'said etching solution and to the surface'to be'etched.

.Preferably isaid etching solution contains from 2 to 40 grams per litre of said finely divided particles.

We have found that there is a relationship bet-ween the concentration of particles in the etching solution, their particulate sizes and the elimination of channelling. More specifically we havefound that the larger the particle'size is, then higher the concentration of the particles inlthe'e'tching' solution must be in order to substantially iel :inatefchanne'lling. It appears that channelling is most effectively reduced by the addition to the etching solution of inert particles of diameters less than 100g.

aid finelyldivided particles may-be selected from the p ice group comprising silicon, silicates, metal oxides, metal carbides, borides, nitrides and carbon.

Said etching solution may comprise an acidic solution of metal ions.

Said etching solution may be based on a mixture of hydrochloric and nitric acids and metal ions present in the following concentrations:

Concentration H 1.0-8.0 Normal. NO 0.5-6.0 Normal. Fe 40-180 g/l. Ni 060 g./l. Cr 040 g/l. Cl ionic concentration of Fe.

According to another aspect of the present invention, a method of etching a partially masked surface comprises exposing the surface to an etching solution comprising a dispersion of finely divided particles, said particles being inert to said etching solution and to the surface to be etched.

Whilst we do not wish to be bound by any theory of the causation of channelling we nevertheless believe the following mechanism to take place:

Channelling appears to be related to the grain size of the articles to be etched, this being particularly true in the case of articles made of nickel and iron based alloys. The grain boundaries of alloys such as these are composed of minor alloy constituents which migrate to the grain boundaries during heat treatment. These minor constituents tend to be more electro-positive the remainder of the alloy. Consequently when these alloys are exposed to acidic solutions, their grain boundaries are preferentially attacked, resulting in the localised evolution of hydrogen.

As will be appreciated, grain size varies from alloy to alloy and thus when considering a unit surface area, some alloys possess a larger surface area of grain bound aries than do others. In the case of alloys having small grains, the surface area of the grain boundaries is comparatively high and consequently the hydrogen ions from the etching solution are able to easily depolarise and evolve as hydrogen gas. The hydrogen ions, however, have more difliculty in depolarising on alloys having larger grains due to the reduced area of the grain boundaries. As a result of the decreased rate of hydrogen evolution, large hydrogen bubbles form in the vicinity of the grain boundaries. These large hydrogen bubbles are difiicult to dislodge and consequently tend to inhibit free circulation of the etching solution, especially in the region of the maskant/alloy interface. Thus as etching proceeds, an insulating barrier of large hydrogen bubbles and corrosion products gradually forms separating the etching solution in the region of the maskant/alloy interface from the remainder of the etching solution. The etching solution so trapped is consequently only in limited heat exchange relationship with the remainder of the etchings'olutioni Thusas etching proceeds, the temperature of the etch ing solution and the article being etched tends to become higher in theregion of the maskant/alloy interface than the remainder of the article and solution. As a result of this localised increased temperature, the rate of attack in this area is correspondingly increased thus resulting in channelling. I 1 T It will be seen therefore that alloys having large grains will be more liable to channelling than those having small grains.

We have discovered that by adding finely divided inert particles to the etching solution, channelling is reduced and may be substantially eliminated. Webelieve that the inert particles are instrumental in preventing the formation of large hydrogen bubbles by providing sites for the formation of a large number of small bubbles. Such bubbles are more easily dislodged from the surface of the workpiece by turbulence than are larger bubbles. We have found that turbulence in the solution which is caused by the etching reaction is sufiicient to dislodge these small bubbles.

As the small bubbles of evolved hydrogen are dislodged from region of the maskant/ alloy interface corrosion products are not trapped and consequently the reaction temperature in this region is similar to that of the remainder of the article and etching solution. As a result of this even thermal distribution the rate of attack of the etching solution in the region of the maskant/ alloy interface is approximately equal to that of the remainder of the exposed alloy surface, thus substantially eliminating channelling.

The finely divided particles may be of any material which is inert to the etching solution and the surface to be etched. Suitable materials include silicon, silicates, metal oxides, metal carbides, borides, nitrides and carbon. These examples illustrate the type of materials suitable and do not constitute a complete list of materials capable of producing the desired effect.

Iron and nickel based alloys are alloys of large grain size which exhibit channelling when partly masked and etched. A typical solution for etching partly masked articles made of nickel and iron based alloys without channelling is as follows:

The following examples will serve to illustrate the invention:

Example 1 An aqueous etching solution was prepared by dissolving an alloy of iron, chromium and nickel in a mixture of hydrochloric and nitric acids and water to give a solution of the following composition:

Concentration H+ 2.7 Normal. NO; 1.8 Normal. Fe 135 g./l. Ni 40 g./l. Cr 30 g./l.

A sample of a nickel based alloy of the following composition:

Percent Mo 6 Ti 2 the remainder being nickel plus impurities was partially masked and etched in the aforementioned etching solution fora period of one hour Whilst maintaining the temperature of the solution at 45 C. After removal from the solution the sample was examined and was found to have severe channelling in the region of the maskant/exposed surface interface.

A further etching solution was prepared as above and to it was added finely divided wood charcoal such that the concentration of the wood charcoal in the etching solution was five grams per litre. The wood charcoal comprised particles of the following sizes:

25% less than 5a 40% less than 10g. less than 30p. 99% less than 80g.

Another partly masked sample of the nickel based alloy was etched in this solution under the same conditions of time and temperature as with the previous sample. 'No channelling was visible on the second sample;

Example 2 Concentration H+ 4.2 Normal. NO 3.1 Normal. Fe g./l.

A sample of the same nickel based alloy as is mentioned in Example 1 was partly masked and exposed to the afore: mentioned etching solution for a period of one hour whilst maintaining the temperature of the solution at 45 C. After removal from the solution, the sample was examined and found to have severe channelling in the region of the maskant/ exposed surface interface.

A further etching solution was prepared as above and to it was added finely divided Kaolin such that the concentration of Kaolin in the etching solution was 40 grams per litre.

Another masked sample of the nickel based alloy'was etched in this solution under the same conditions of temperature and time as with the previous saniple.' No ch-annelling was visible on this sample.

Example 3 A partly masked sample of the nickel based alloy described in Example 1 was etched in this solution under the same condition of temperature and time as in Example! with no visible channelling.

Example 4 An aqueous etching solution was prepared as described in Example 2 and to it was added finely divided aluminium oxide such that the concentrationof aluminiu oxide in the solution was 15 grams per litre.

A partly masked sample of the nickel based alloyzdee scribed in Example 1 was etched in this solution under the same conditions of time and temperature as described in Example 1 with no visible channelling. I

Example 5 An aqueous etching solution was prepared as described in Example 2 and to it was added finely divided wood charcoal such that the concentration of wood charcoal in the solution was 10 grams per litre. The woodcharc'oal was of a similar particle size range asthat'deScribed in Example 1. V j r A partly masked sample of the nickel based alloyldescribed in Example 1 was etched in this solution under the same conditions of time and temperature as in Example l with no visible channelling.

Example 6 An aqueousetching solution was prepared by dissolving an al,ly 0f;iron, nickel and chrominum in a mixture of nitric acid and hydrochloric acids. The free acid .and nitrate concentrations in the solution were then adjusted to the following levels by further additions of nitric and hydrochloric acids:

A partly masked sample of the nickel based alloy described in Example 1 was etched in the aforementioned solution under the same conditions of time and temperature as in Example 1. After removal from the solution, the sample was examined and found to have severe channelling.

A further etching solution was prepared as above and to it was added finely divided wood charcoal such that the concentration of the wood charcoal in the etching solution was 10 grams per litre. The wood charcoal was of a similar particle size range as that described in Example 1.

A partly masked sample of the nickel based alloy described in Example 1 was etched in this solution under the same conditions of time and temperature as in Ex ample 1 with no visible channelling.

Example 7 An aqueous etching solution was prepared by dissolving ferric chloride and ferric nitrate in a mixture of nitric and hydrochloric acids and water to give a solution of the following composition:

A partly masked sample of the nickel based alloy described in Example 1 was etched in this solution under the same conditions of time and temperature as described in Example 1. Upon removal from the etching solution the sample was examined and found to have severe channelling."

A further etching solution was prepared as above and to it was added finely divided particles of aluminium oxide and graphite such that the concentration of the aluminium oxide was grams per litre and the concentration of the graphite was 5 grams per litre in the etching solution.

A partly masked sample of the nickel based alloy described in Example 1 was etched in this solution under the same conditions of temperature and time as in Example 1 with no visible channelling.

Example 8 An aqueous etching solution was prepared by dissolving a mixture of ferric chloride and ferric nitrate in a mixture of nitric and hydrochloric acids and water to give A partly masked nickel based alloy of the following composition:

the remainder being nickel plus impurities was etched in this solution under the same conditions of temperature and time as described in Example 1. Upon removal from the etching solution, the sample was examined and found to have severe channelling.

A further etching solution was prepared as above and to it was added finely divided wood charcoal such that the concentration of the wood charcoal in the etching solution was 15 grams per litre. The wood charcoal was of a similar particle size range as that described in Example 1.

A further partly masked sample of the aforementioned nickel alloy was etched in this solution under the same conditions of time and temperature as in Example 1 with no visible channelling.

Example 9 Concentration H+ 4 Normal. NO 2 Normal. Fe 150 g./l.

A partly masked sample of a nickel based alloy of the following composition:

Percent Fe 35 Cr 16 Mo 3 Al 1 Ti 1 the remainder being nickel plus impurities was etched in this solution under the same conditions of temperature and time as in Example 1. Upon removal from the solution the sample was examined and found to have severe channelling.

A further etching solution was prepared as above and to it was added finely divided wood charcoal such that the concentration of wood charcoal in the etching solution was 15 grams per litre. The wood charcoal was of a similar particle size range as that described in Example 1.

A further partly masked nickel based alloy sample was etched in this solution under the same conditions of temperature and time as Example 1 with no visible channelling.

We have found that before etching commences, it is desirable to air agitate the etching bath in order to ensure an even distribution of particles throughout the solution. After etching has commenced, the turbulence produced by the etching reaction is normally sufiicient to maintain the particles in suspension.

Although the invention has been described with reference to acidic etching solutions based on hydrochloric and nitric acids, it will be appreciated that other acidic as well as alkaline etching solutions which contain inert particles could be successfully used for etching large grained partly masked alloys with reduced channelling.

We claim:

1. An etching solution comprising a dispersion of finely divided particles less than p. in diameter and selected from the group consisting of silicon, silicates, metal oxides,

metal carbides, borides, nitrides and carbon, the particles 3 I References- Cited 5 being inert to the etching solution and to the surface to V I I ,7 be etched, and including hydrochloric acid, nitric acid UNITED STATES PATENTS and metal ions present in the following concentrations: 3,033,793 5/1962 Bradley t 1 156 18XR C t v 5 3,235,426 2/1966 Bruner 156 -2 7 3,043,362 7/1962 Menne'sson' 156 18X R H+ to Normal- 3,330,743 7/1967 Jestl et=al.'- ."156-1'2'1 'XR NO; 0.5 to 6.0 Normal. 3,479,293 11/1969 "Bellinger et a1. -43-- 252--79;4 Fe 40 to 180 grams per litre. f Ni Q"; m 0 to 60 gr s p litre- 10 WILLIAM A. POWELL, Primary Examiner Cr 0 to 40 grams per litre. US. Cl. X.R.

Cl ionic concentration of Fe. 156-7, 18 

